The present invention relates to a ring laser angular rate sensor, usually referred to as a ring laser gyroscope. More particularly it relates to a readout apparatus for such a ring laser gyroscope.
A ring laser gyro is a laser apparatus having a ring type resonant cavity, or simply a ring resonator. The ring resonator is commonly constructed of a block having a plurality of interconnecting tunnels in the shape of a polygonal path such as, for example, a triangular or rectangular path. The laser beam is directed around the ring path by suitable mirrors appropriately positioned at the intersections of pairs of interconnecting tunnels. In ring laser gyros there are commonly two laser beams traveling in opposite directions relative to each other around the polygonal ring path. The positioning of the mirrors in the corners of the polygonal ring path direct the laser beams through the tunnels of the resonant cavity. At one of the corners, the mirror is partially transmissive whereby major portions of the counter-propagating beams are reflected, while small portions of each of the counter-propagating beams are transmitted through the mirror into a readout assembly. Some examples of ring laser gyros are shown and described in U.S. Pat. Nos. 3,373,650 and 3,467,472 issued to Killpatrick, and U.S. Pat. No. 3,390,606 issued to Podgorski, these being incorporated herein by reference.
The aforementioned readout assembly generally comprises a prismatic structure for combining those small portions of each of the counter-propagating beams to produce either an interference fringe pattern comprised of light intensity bands with directional movement, or alternatively a light intensity spot which varies in intensity between high and low values at a rate proportional to the frequency difference between the counter-propagating beams. The readout assembly may be either "block mounted" or displaced from the block. U.S. Pat. No. 3,373,650 illustrates a readout assembly which is displaced from the gyro block. In U.S. Pat. No. 3,373,650, the readout assembly is comprised of a corner prism which combines the small portions of the counter-propagating beams which are transmitted through the partially transmissive mirror, and recombines them to form an interference fringe pattern.
U.S. Pat. No. 4,582,429, issued to Steven P. Callaghan, U.S. Pat. No. 4,677,641 issued to Theodore J. Podgorski, and U.S. Pat. No. 4,712,917 issued to Bergstrom et al. all show block mounted readout assemblies which are all solid structures comprised of one or more prism elements. These prism elements combine those portions of the counter-propagating laser beams, transmitted through the partially transmissive mirror, to produce an interference fringe pattern or light intensity spot. These patents are also incorporated herein by reference.
The aforementioned Callaghan and Bergstrom et al. Patent disclose prismatic structures which combine the small portions of the counter-propagating laser beams to produce an interference light spot. The Podgorski Patent shows a unitary solid structure prism for producing an interference fringe pattern; and the Callaghan and Bergstrom et al. Patents show a solid structure prism comprised of a pair of prisms mounted to a mirror assembly substrate. All of the just referred to readout assemblies are all intended to be directly mounted to the ring laser gyro block.
Associated with the readout assemblies of the prior art is at least one photodetector which is generally responsive to either the interference fringe pattern or the interference light spot.
Readout assemblies known in the prior art for ring laser gyros are particularly susceptible to radiation-induced noise, i.e., unwanted signal or effect generally caused by nuclear radiation, which may result in poor sensor performance due to rotation rate information errors. The magnitude of the sensitivity to radiation-induced noise is thought to be directly related to the size of the photodetectors. The size of the detectors being determined by the size of the output light spot or interference fringe pattern.